Tankage capability is an important ingredient of any industrial facility handling large quantities of liquid fuels, e.g., a petroleum refining plant processing hundreds of thousands of barrels of oil a day. To insure continuing uninterrupted operation, the facility must contain means for storing raw materials (e.g., crude petroleum feed) and products manufactured in the facility. One of the oldest storing means used in the petroleum industry are fixed roof tanks usually having conically-shaped roofs. However, it has been found that the fixed roof tanks allow substantial amounts of vaporized materials to escape from the tanks due to the phenomena called "breathing", and to losses occurring during filling and withdrawal of materials from and into the tank. Breathing occurs when constantly changing cycles of temperature force contraction of the gas volume on cooling and consequent inhalation of air into the tank. Conversely, an increase in temperature expands the gas volume, forcing the vapor-saturated air to flow out through the tank vents. After each cycle, the vapor phase of the tank comes into an equilibrium with the liquid stored therein. Upon exhalation, at least some of the light ends of the stored products are withdrawn from the tank. Although some conservation means have been designed for the fixed roof storage tanks, e.g., pressure-vacuum valves in the vents of roof tanks, floating roof tanks have found increased acceptance and popularity in the industry.
A floating roof tank typically comprises a substantially cylindrical tank having a roof floating on the liquid contents of the tank. The most popular roof designs are pontoon and double pontoon roofs. A floating roof tank substantially reduces the losses of vapors from above the liquid level because the vapors can escape only from a small peripheral space between the wall of the tank and the floating roof. However, due to constantly increasing cost of petroleum products and crude, and due to environmental considerations, eliminating or substantially decreasing even the small losses from the peripheral area is becoming an important consideration.
Attempts have been made in prior art to decrease such losses by providing the floating roofs with flexible seals extending from the roof outwardly toward the tank wall. However, it has been found that protrusions on the inside wall of the tank, e.g., rivets, bolts and buttstraps allow the escape of some of the vapors even through such seals. The protrusions also damage the seals, thereby increasing maintenance and capital costs associated with roof tanks having such seals. Accordingly, attempts have been made in prior art to use a double seal construction comprising a primary seal made of a flexible material and extending from the floating roof to the vertical inner wall of the tank, and an auxiliary seal super-imposed above the primary seal and constructed of a plurality of flexible sheet members above the primary seal (e.g., Wadwell et al, U.S. Pat. No. 4,099,643). Thus, the auxiliary seal and the primary seal form a vapor space which decreases the amount of vaporized products able to escape from the tank when the auxiliary seal encounters an obstacle in the inside wall of the tank. However, even this double-seal design has also presented some maintenance problems insofar as the constant contact thereof with the obstacles on the inside wall of the tank may cause damage to the seal, thereby necessitating periodic replacement thereof.
Attempts have also been made in prior art to provide tanks using only a single seal construction with a roller, in some designs fastened to a tensioned support, and placed above the seal (see, e.g., British Patent No. 487,774, Hammeren, U.S. Pat. No. 2,180,587 and Haupt, U.S. Pat. No. 2,190,476). The roller rides on the inside surface of the tank to guide the movement of the floating roof and to ride over projections of the inside wall. However, the prior art designs incorporating the roller exhibit several deficiencies which allow the vapors to escape from above the liquid surface. For example, the rollers, due to their angle of inclination with respect to the wall are not pressed against the wall with sufficient force, nor with uniform pressure, thereby allowing some vapors to escape into the atmosphere. The rollers of the prior art also do not compensate for variations in the diameter of the tank, or for irregularities of the inside of the tank shell, nor do they provide sufficient pressure to keep the entire circumference of the roof in substantially constant contact with the inside tank wall. Additionally, the prior art roof seal designs incorporating the rollers use them in conjunction with a single seal construction, thus allowing relatively large quantities of valuable vapors to escape into the atmosphere. The prior art roof seal designs also do not satisfy increasingly stringent environmental requirements which mandate very low levels of hydrocarbons emissions and a complete separation of primary and secondary seals.
Accordingly, it is a primary object of this invention to provide an improved sealing means for floating roof storage tanks.
An additional object of this invention is to provide a floating roof storage tank with an improved sealing means comprising a roller inclined with respect to the vertical surface of an inner wall of the tank at a specific angle, thereby effectively preventing substantially all of the vapors from escaping into the atmosphere.
It is an additional object of this invention to provide a floating roof storage tank utilizing a double-seal construction with a roller means inclined at a specific angle with respect to the vertical inner wall of the tank, thereby preventing the obstructions in the tank wall, e.g., rivets and buttstraps, from damaging the double-seal of the floating roof tank and compensating for irregularities of the inside wall diameter by providing constant and continuous pressure on the secondary seals.
Additional objects of this invention will become apparent to those skilled in the art from the study of the following specification and appended claims.